More specifically, a particular parabolic flight, called “0 g” flight, g being the acceleration of gravity, makes it possible to generate, for some tens of seconds, a 0 acceleration (in all directions, although it is the vertical direction that is primarily considered in the context of the present invention) and therefore the conditions of weightlessness in the aircraft. Such a flight is, generally, provided with a tolerance of 5/100th g about 0 g.
A flight director has been developed to guide the pilots between the pitch-down and pull-up actions during such a 0 g flight. This flight director indicates a predefined sought position of the (control) column to follow a parabolic trajectory at 0 g. This predefined position of the column is computed, before the flight, as a function of parameters linked to the aircraft, such as the weight, the centering, the balance, aerodynamic parameters, the thrust, etc.
This flight director only makes it possible to compute an estimation of the parabolic trajectory generating a 0 g flight and does so for various reasons:                it is not possible to know the different parameters of the aircraft accurately enough to compute the trajectory with the desired accuracy; and        the factors external to the aircraft which influence the load factor (for example movements of the air) are not taken into account.        
This flight director therefore indicates a position of the column which may present a slight offset with the position making it possible to perform a precise 0 g flight. The pilot is therefore also obliged to follow a load factor indicator (g-meter) in addition to this flight director, to perform a parabolic flight.